Toxoplasma

Toxoplasma gondii is a species of parasitic protozoa, belonging to the Apicomplexa, that can cause the disease toxoplasmosis in humans. The life cycle of Toxoplasma gondii has two phases. The sexual part of the lifecycle take

Toxoplasma gondii is a species of parasitic protozoa, belonging to the Apicomplexa, that can cause the disease toxoplasmosis in humans. The life cycle of Toxoplasma gondii has two phases. The sexual part of the lifecycle takes place only in cats, the definitive host of Toxoplasma gondii. The asexual life cycle can take place in any warm-blooded animal, like other mammals and birds. In these intermediate hosts, the parasite invades cells forming a so-called parasitophorous vacuole. Within this vacuole T. gondii propagates by a series of binary fissions until the infected cell bursts. This fast replicating form of the asexually reproducing T. gondii is called tachyzoite. Additionally, a slow replicating form exists, the bradyzoite. Bradyzoite vacuoles can form tissue cysts in the infected animals/humans (mainly within the muscles and brain). Resistance to antibiotics varies, but the cysts are very difficult to eradicate entirely. The immune system of the host does not detect these cysts, while free tachyzoites are efficiently cleared by the immune response.

Tissue cysts can be ingested by a cat, e.g. by feeding on an infected mouse. The cysts survive passage through the digestive system of the cat and the parasites infect epithelial cells of the small intestine where they undergo sexual reproduction and oocyst formation. Oocysts are shed with the feces. Animals and humans that ingest oocysts (e.g. by eating unwashed vegetables etc.) become infected. The parasite enters macrophages in the intestinal lining and is distributed via the blood stream throughout the body.